1. Field of the Invention
The invention relates to the field of automated analysis of workpieces for optically detectable flaws, and in particular to the optical detection and assessment of sidewall and edge flaws in cylindrical nuclear fuel pellets.
2. Prior Art
In the manufacture of nuclear fuel rods, nuclear fuel pellets are formed from a matrix of enriched or natural uranium oxide and are inserted into elongated hollow tubes typically made of an alloy of zirconium, to be used as fuel rods. The pellets are short cylinders and are stacked in the tubes. The tubes are sealed at the ends with end plugs, and pressurized. A plurality of fuel rods are arranged in an array to form the fissionable core of a nuclear reactor.
Defects in the pellets, such as fissures or cracks, can result in chips becoming dislodged from the pellets during reactor operation. Such chips can adversely affect the operation of the reactor. For example, a chip dislodged from a fuel pellet can become lodged within the fuel rod adjacent the interior wall of the rod. The fissionable material in the dislodged chip continues its fission reaction under the influence of nuclear flux in the reactor core. Heat generated at the chip can create a localized area of intense heat on the wall of the fuel rod. This weakens the rod at the point of the chip and may cause a rupture of the rod wall and a leak of high pressure gas from the rod. If this occurs, the reactor may have to be shut down.
Other pellet defects, which may be visible at the surface of the pellets, also can cause adverse effects. For example, the inclusion of metal in the pellet surface may adversely affect the normal fission reaction of the uranium. The pellets must be free of defects such chips, cracks, etc., for optimal operation of the reactor. Conventionally, pellets are inspected visually by human inspectors. Specially dressed inspectors visually examine illuminated trays of lengthwise stacked pellets and manually remove any that are apparently defective. After one side of the pellets is viewed, the trays are covered and flipped over, for viewing the opposite side.
The results of manual inspection of the pellets in this manner are inconsistent due to the subjectivity inherent in human visual inspection. Moreover, manual inspection requires prolonged exposure of the inspectors to the low level radiation produced by the pellets, as well as potential ingestion of dust which the pellets produce. Also, the trays are heavy and physically manipulating them is strenuous and presents a risk of injury.
U.S. patent application Ser. No. 07/640,770, filed Jan. 14, 1991 and entitled Pellet Inspection System, discloses an automated transport arrangement for presenting pellets serially for inspection using an automated viewing apparatus. The disclosure of said Application is hereby fully incorporated. According to the disclosure, the pellets are fed along a conveying means that rotates and axially feeds the pellets as they pass in front of a line scanning camera. The image of the pellets is digitized as a series of sets of discrete digital values representative of axially extending linear portions covering the peripheral surface of each passing pellet. The digital values, which represent the reflectivity of the pellets in a small area or pixel according to a numerical gray scale, are collected and their values are averaged. High and low reflectivity thresholds are defined from the average, and the pixel values are compared to the thresholds thus defined. The number of pixels that fall outside the threshold are counted, and if the number exceeds a predetermined value the respective pellet is rejected. The system is arranged such that the pellets are axially separated from one another as they are fed, whereby it is also possible to use the system to check for proper pellet dimensions without the need to detect the axial ends of the pellet.
Additional examples of optical inspection systems for cylindrical pellets and the like are disclosed in U.S. Pat. Nos. 4,496,056 and 4,549,662--both to Schoenig, Jr. et al, and U.S. Pat. No. 4,448,680--Wills et al.